An example of a microphone array including a plurality of microphones arranged in array is disclosed in Patent Literature 1. According to Patent Literature 1, first and second microphones are disposed with a predetermined distance D therebetween in a microphone array. A third microphone is disposed between the first and second microphones, being spaced by a distance D/2 from the first and second microphones. A fourth microphone is disposed at a location between the third and first microphones, being spaced by a distance D/4 from the first and third microphones. A fifth microphone is disposed at a location between the third and fourth microphones, being spaced from the third and fourth microphones by a distance D/8. A sixth microphone is disposed at a location between the fifth and third microphones, being spaced from the third and fifth microphones by a distance D/16. A seventh microphone is disposed at a location between the sixth and third microphones, being spaced from the third and sixth microphones by a distance D/32. A eighth microphone is disposed at a location between the seventh and third microphones, being spaced from the third and seventh microphones by a distance D/64.
Audio signals from these microphones, which form microphone units, are processed by beam forming. More specifically, the first through third microphones form a first microphone unit; first, fourth and third microphones form a second microphone unit; the third through fifth microphones form a third microphone unit; and the third, fifth and sixth microphones form a fourth microphone unit. The third, sixth and seventh microphones form a fifth microphone unit; and the third, seventh and eighth microphones form a sixth microphone unit. The audio signals from the first and second microphones in the first microphone unit are multiplied by a factor 0.5, and the audio signal from the third microphone is multiplied by a factor 1. The audio signals from the first through third microphones multiplied by the respective factors are combined by combining means into a composite signal. Similarly, in each of the second through sixth microphone units, the audio signals are processed in such a manner that the audio signals from the microphones located at the opposite ends of that microphone unit are multiplied by a factor of 0.5 with the audio signal from the microphone at the center of that microphone unit being multiplied by a factor 1, whereby a composite signal is formed. The thus produced composite signals are combined to thereby provide the microphone system with sharp directivity over a wide frequency range. Further, the audio signal from each of the microphones is provided with a delay in a delay circuit, and the delayed audio signals are supplied to a beam forming section. With this arrangement, influence of differences in distance between a speaker and the respective microphones is removed by providing a delay equal to the delay of the last arriving audio signal for the audio signals from the other microphones.